DRO Pros is what they specialize in. QMT/PM has great customer service & that is a big plus. Not saying DRO Pros doesn't, I hear they are good people too but I have never dealt with them. I have dealt with Matt at QMT a lot & he bends over backwards to make you happy.
I don't think one or the other would be easier to install except for maybe glass vs mag scales (mag scales are slimmer & can be cut easier). I have glass scales on both my lathe & mill & they weren't hard to install, just time consuming.
Both sell popular brands like Easson. DRO Pros has their own branded units as does PM now. They're all made in China. I have Easson scales & display on my mill, purchased from QMT/PM. Has never gave me a problem. I have Sino scales on my lathe & a M-DRO display on my lathe, again, never gave me problem. Had them for 10 yrs now. One of the PM displays that are available now is pretty much the same as the M-DRO display I have. They used to be sold under the brand name Meister.
Shouldn't have a problem with either vendors. I personally would buy from QMT/PM just cause Matt has always taken care of me & if I ever had an issue I know I would easily be taken care of. Cheaper than DRO Pros too. But DRO Pros has a bigger selection to choose from.